Many offshore structures such as drilling rigs and production platforms for oil and gas segregate run off water from rainstorms and wash down water used to clean equipment and deck surfaces from produced or process water. The rainwater and wash down water may pick up oil and or grease that would be harmful to aquatic life if discharged into the surrounding body of water without treatment. Before discharging into a surrounding body of water, it is segregated and treated separately from produced or process water to avoid overloading treatment systems or causing unwanted solids formation. Government agencies regulating this discharge occasionally promulgate more stringent oil and grease discharge requirements for the discharge of these wastewater streams, which require more reliable treatment and better treatment methods.
Many disadvantages are associated with current wastewater sump tanks, disposal caissons or piles and their methods of use. One disadvantage is ineffective separation of the oil and grease droplets of the particle size typically found in rainwater runoff and/or wash down water. Ineffective separation may also be caused by improper design criteria such as inadequate residence times, rainwater runoff projections or by the physical design of the tank, caisson or pile. One physical design disadvantage noted is that the downward velocity component of the exiting wastewater into the bottom of the caisson may carry the oil droplets outside the lower end of the caisson before the separation of the oil from the water may be accomplished. Thus, oil droplets entrained in the water escape into the surrounding body of water before the droplets can rise to the surface. The location of the wastewater exit into the caisson at the lower end exacerbates this effect. This exit location allows interference with the oil and water separation by tidal and wave actions. Additionally, adding multiple appurtenances to the inside of the caisson increases the downward velocity, reduces retention time and provides areas for clogging and unnecessary shearing of oil particles.
Another disadvantage of current wastewater disposal systems is the use of sump tanks that are prone to fill up with solids. Many current designs use an overflow weir positioned at the top of the vapor/oil interface. The solids settle to the bottom of the sump tank. The accumulation of solids in the sump tank may result in severe oil removal problems that require the unit to be shut down for cleaning. Additionally, maintenance of fouled pipes or pump repairs may be required. Current designs may also have oil sumps prone to fill with water or "wash out" when precipitation rates are high. Oil discharged with the water exceeds the mandated discharge requirements and water discharged from the oil sump may be transported through recovered oil lines and contaminate recovered oil storage. Further treatment of the resulting mix of oil and water may then be required.
Another disadvantage is escaping hydrocarbon vapors from the treatment apparatus. Hydrocarbon volatilized from the separated oil above the vapor/oil interface inside the tank, caisson or pile may form potentially explosive mixtures inside the apparatus. Leaks of the mixture outside the apparatus may drift to potential spark or ignition sources found on or near the platform with disastrous results.
Other disadvantages of current treatment tanks, piles or caissons presently exist. These may include not being able to adjust overflow weirs or slots that skim the accumulated separated oil from the treated water. Additionally, inadequate protection from corrosive marine environments, problems with evacuating the accumulated oil from the oil sumps and nonexistent or inoperable sampling of the treated wastewater are noted in current designs.
Therefore, the art has sought a method and apparatus for treating rainwater and wash down water to more effectively separate oil and grease droplets from the wastewater before its discharge into the surrounding body of water and which reduce maintenance, process safety and environmental concerns.